1. Field of the Invention
The present invention relates to an intrusion warning system utilizing an antenna on which an A.C. voltage is impressed.
2. Description of the Prior Art
FIG. 1(a) is a block diagram showing an example of the conventional intrusion warning system utilizing an electric field, such as shown in the U.S. Pat. No. 4,064,499. In the system of FIG. 1(a), a voltage of a high frequency, for example, about 9750 Hz shown by FIG. 2(E) and generated by a signal generating circuit E is applied across a field wire S and the ground, and a voltage change induced on an antenna A disposed with a substantially uniform distance gap g from the field wire S is detected and processed to produce an intruder detecting signal. The voltage change due to the intruder is detected as follows:
Provided that a resistance between the antenna and the ground, i.e. an input impedance of the amplifier is sufficiently large, by considering the equivalent circuit of FIG. 1(b), the induced voltage V for the case that there is no intruder, is given by: ##EQU1## wherein
C.sub.1 ' . . . is a capacitance between the field wire S and antenna A,
C.sub.o . . . is a capacitance between the antenna A and the ground,
e . . . is a voltage of the high frequency signal applied to the field wire S.
Then, by considering the intruder's body as an electric conductor, and capacitance between the intruder's body and the antenna A is C.sub.M, the induced voltage V.sub.a ' becomes as shown by the following equation (2): ##EQU2##
The induced voltage is amplified by an amplifier 7, then detected by a detection circuit 8 as FIG. 2(a)(V.sub.d), and after passing a band-pass filter 9 led to a threshold circuit 10, where its input value V.sub.a ' is compared with a predetermined threshold value V.sub.th.
When a man passes through the electric field of the antenna A as shown by FIG. 1(c), the induced voltage V.sub.a ' changes as shown by FIG. 2(V.sub.a ') by the change of the capacity C.sub.M, and therefore, the input voltage V.sub.a to the threshold circuit 10 changes as shown by FIG. 2(V.sub.a) and by FIG. 1(d). Therefore, when the input voltage V.sub.a becomes lower than the predetermined threshold value V.sub.thL, at the time t.sub.1 as shown by FIG. 2(V.sub.a), the threshold circuit 10 sends an output signal to the warning circuit 6, which issues a warning signal at the time t.sub.1 as shown by FIG. 2(V.sub.s), to light a lamp or ring a buzzer.
The sensitivity of the detection is largely influenced by changes of the capacitance C.sub.o between the antenna wire S and the ground which is dependent on the length of the antenna wire S and height of the antenna wire S from the ground. Therefore, the conventional system has had a problem that the apparatus can not necessarily has a sensitivity as designed, since the design is made for an average antenna wire of average height and length, or in other words, the use of the apparatus is only for a limited range of height and length of the antenna. Moreover, to provide a pair of wires may have a difficulty in actual case because of surrounding conditions.